1. Field of the Invention
The invention relates to a device for etching semiconductor wafers having a large surface area.
2. Description of the Prior Art
It is advantageous or unavoidable for a large number of novel products or working steps in the field of semiconductor technology to use electrochemical etching methods in addition to chemical methods. This more particularly applies to the production of a large variety of pores in e.g. silicon, GaAs, InP or GaP, which can only be produced electrochemically.
Generally for such etching methods, an anodic current is passed through the semiconductor (i.e. the positive pole of the current supply at the semiconductor), which induces at the semiconductor-electrolyte transition a chemical reaction leading to the dissolving of the semiconductor material. The structure to be produced, e.g. so-called macropores with diameters of around 1 μm and depths of a few 100 μm, must be homogeneous over the entire semiconductor surface, and in addition, a simple, reliable and rapid process is highly desired.
Electrochemical (pore) etching in silicon is typically used in technical fields such as:                microelectronics and microsystem,        biotechnology, e.g. biochips or biosensors,        sensor means in general,        production of so-called SOI (Silicon On Insulator) wafers,        production of photon crystals, special filters and quantum optics or non-linear optics elements,        solar means (e.g. for producing antireflection coatings),        fuel cells (as porous electrode),        nanotechnology, e.g. in the production of nanowires.        
These applications lead to demands which are very difficult to fulfill for large area semiconductor wafers, e.g. silicon wafers with diameters of 300 mm. Even for smaller samples or specimens with surfaces of a few cm2, it is not readily possible to achieve a homogeneous etching. The difficulties arise through the combination of many special circumstances, which with surface areas larger than a few cm2 in all, very rapidly lead to the limits of conventional etching cells. In particular, the following factors are critical and must be respected:                Homogeneous outward and return transport of the electrolyte to the reactive interface. In the case of simple, electrolyte-scavenged cells the flow pattern of the electrolyte always give rise to etching inhomogeneities.        Homogeneous electrical contact with the back of the semiconductor. This contact must be able to bear large currents. With etching current densities of up to about 100 mA/cm2, for typical Si wafers with surface areas of 100 cm2 there are total currents of 10 A and higher, which can lead to electrical and thermal problems.        Possibility of homogeneous illumination of the back surface with high intensity light, which is required for many applications.        Temperature control and monitoring within narrow limits (without influencing the optimized flow behavior).        Absolute tightness of both the wafer mounting and the entire apparatus.        No wafer breaks, also not in the case of samples which have become highly porous through etching and which, therefore, have become mechanically very susceptible to problems.        Resistance of all materials wetted with the electrolyte with respect to extremely aggressive chemicals (e.g. mixtures of HF and organic, high polar solvents).        Safe removal of large amounts of gases which can arise during etching (generally H2 and O2, but in certain cases also the extremely toxic gases PH3 and AsH3).        Usability for all types of semiconductors, e.g. n and p-doped silicon, GaAs, InP, etc.        
Known devices for etching n-silicon in aqueous electrolytes already have an illumination of the back surface, the electrolyte flowing over the vertically installed wafer. Electrical back surface contact is made possible by a special n+ implantation of the back surface and contact needles at the wafer edge. This has hitherto made it possible to produce so-called n-macropores (aqu/bsi) with depths of up to 600 μm, but use for other semiconductors, e.g. p-type silicon or InP is not readily possible.
It is also known to use as the back surface contact a second electrolyte, and hydrogen is formed.
Finally, on an industrial scale in the production of SOI substrates, use is made of a device for etching so-called mesopores in silicon. However, etching depths of roughly only 10 μm are required, i.e. the method is much less demanding than the homogeneous etching of macropores with extreme aspect ratios.